2.1. BREEDING GOALS IN TOMATO
Tomatoes produced in the United States are typically used for a variety of different purposes, and therefore, development of different varieties having features adapted for a particular end use, is one of the major goals of tomato breeders. For example, the largest proportion of tomatoes produced are used for processing, e.g., for juices, soups, sauces or other tomato-based products in which the tomato undergoes some degree of alteration before being presented to the consumer. A smaller, but significant portion of the overall tomato crop produced in the United States are fresh market tomatoes, i.e., those which appear on supermarket shelves and are purchased as fresh produce by the consumer.
Needless to say, the qualities which would be desirable in a processing tomato are not necessarily those which would be desirable in a fresh market tomato; thus, the selection process for suitable varieties for each specific end use will necessarily be different as well. Of course, certain traits, such as disease and pest resistance, high yield and concentrated fruit set are of interest in any type of tomato line. However, certain features, such as solids content, which are the constituent components of tomato fruit other than water, and small and firm fruit (to facilitate mechanical harvesting) are more actively sought in development of processing tomato lines.
On the other hand, superficial, external features such as intensity and uniformity of fruit color, unblemished fruit, and larger but uniform fruit size are typically more important to the development of a fresh market product which will have consumer appeal. Intensely and uniformly red tomatoes in the prior art have been found to contain the crimson (og.sup.c) and/or high pigment (hp) allele (Sayama and Tigchelar, 1985, Japan J. Breed. 35:145-152). Attempts to obtain intensely red tomatoes have had only limited success. Although high-pigment tomatoes are not unknown, they have been largely restricted to use as home-grown varieties, and are not available on a large-scale, commercial basis, primarily because of the relatively low yields that tend to be associated with this trait. This is particularly true of tomatoes carrying the hp allele, which produces an excellent red color, but which also has a number of undesirable pleiotropic effects associated with it, such as slow germination and growth, and premature defoliation.
Efforts to obtain new variants having the desired characteristics as the basis for development of new varieties has to a large extent focused on attempts to obtain interspecific hybrids, particularly with the wild species L. chilense, and L. peruvianum. These two species have been thought to hold a great deal of promise in development of new cultivated varieties, as they both contain a number of valuable traits which would theoretically prove useful in improving the cultivated variety. L. peruvianum, for example, is known to contain genes for resistance to numerous tomato diseases and root knot nematode, as well for high vitamin C content. Unfortunately, obtaining hybrids between the cultivated tomato and the wild species L. peruvianum has been severely hampered by strong interspecific incompatibilities. Through hybridization by traditional methods between these two species, fruits are only obtained if L. esculentum is the female parent, and even with this control, it is necessary to employ embryo culture to establish seedlings. The laborious and tedious manipulations required with hand pollination and embryo culture have thus far prevented traditional breeding methods from being a valuable means of obtaining genetic variation in L. esculentum.